Conventionally used is a flexible wiring board having a wiring pattern on a flexible insulating substrate which is made of polymer such as polyimide for connection between various electrical components, in particular, between a liquid crystal display element and a driving circuit.
An example of the conventional flexible wiring board will be described based on FIG. 5 which shows a cross section thereof. As shown in FIG. 5, by a copper foil adhesive layer 105, a copper foil pattern 102 is bonded with a base polymer film 101 which is a flexible insulating substrate made of polymer (flexible substrate). The adhesive used for the copper foil adhesive layer 105 includes epoxy resin or phenol resin, etc.
Further, the copper foil pattern 102 is covered with an insulative protecting film 104 made of polymer, and the insulative protecting film 104 is bonded with the copper foil pattern 102 by an insulative protecting film adhesive layer 106. However, an end portion of the copper foil pattern 102 is not covered with the insulative protecting film 104 and is exposed to function as a terminal portion to be connected to an external electrical component.
The insulative protecting film 104 has the functions of insulating the copper foil pattern 102 from outside, protecting the copper foil pattern 102 from corrosion such as rust, and increasing folding endurance of the flexible wiring board. As the material of the insulative protecting film 104, polyimide is commonly used.
On the surface of the exposed portion (terminal portion) of the copper foil pattern 102 is formed a plated layer 103 by Au/Ni plating (Au plating over underlying Ni layer) or Sn plating so as to stabilize the connection to the external electrical component by preventing rust on the copper foil pattern 102.
Note that, even though FIG. 5 shows the arrangement where the base polymer film 101 and the copper foil pattern 102 are bonded by the adhesive (copper foil adhesive layer 105), the arrangement where the base polymer film 101 and the copper foil pattern 102 are directly bonded with each other without using the adhesive, i.e., an adhesive-less flexible wiring board has been used as well.
In response to the recent reduction in external size of various electrical devices, there has been strong demand for efficient use of space for mounting components of these devices. Accordingly, there is need to bend the flexible wiring board which is connected to a connection terminal of an electrical component such as a liquid crystal display element and which supplies to the connection terminal various signals such as inputs from other electrical components which are disposed independently (separately) from the above electrical component, so as to avoid interfering the mount of the other electrical components and to reduce the size of the entire device including these electrical components, depending on where the device is disposed.
Especially, when the conventional flexible wiring board is to be connected to an end portion of the electrical component such as the liquid crystal display element, it is required to bend the flexible wiring board at the position as close as possible to the end portion of the electrical component.
FIG. 6 shows a cross section of the conventional flexible wiring board in a state where it is connected to an end portion of a liquid crystal panel mounting an IC (Integrated Circuit) chip 122 thereon by the chip-on-glass method (“COG” method hereinafter), and is bent 90° in a direction (downward in FIG. 6) which would make the connected surface to face inward.
As shown in FIG. 6, the flexible wiring board 110 has the same layer structure as the flexible wiring board of FIG. 5, and includes a base polymer film 111, copper foil pattern 112, plated layer 113 which is plated, for example, by Au/Ni plating, insulative protecting film 114, copper foil adhesive layer 115, and insulative protecting film adhesive layer 116, which are analogous to the base polymer film 101, copper foil pattern 102, plated layer 103, insulative protecting film 104, copper foil adhesive layer 105, and insulative protecting film adhesive layer 106, respectively.
Meanwhile, the liquid crystal display element 120 is arranged such that a liquid crystal layer 126 is filled between a pair of glass substrates 121 and sealed by a sealant 125 therebetween, and one of glass substrates 121 (lower glass substrate 121 in FIG. 6), which is larger than the other glass substrate 121 (upper glass substrate 121 in FIG. 6), extends beyond the upper glass substrate 121.
The inner surface of the extending glass substrate 121 is connected to an IC chip 122 which generates a video signal or driving signal for driving the liquid crystal (liquid crystal layer 126) based on external input signals, and on an end portion on the inner surface of the extending glass substrate 121 is laid an end portion of the flexible wiring board 110 so that a connection terminal of the flexible wiring board 110 (portion where the plated layer 113 is provided) is connected thereto.
Further, there are provided a connecting section 124 which includes an input electrode for inputting signals to the IC chip 122 and a webbed wiring pattern for supplying signals to the input electrode from the flexible wiring board 110, and a connecting section 123 which includes an output electrode for outputting the video or driving signal from the IC chip 122 and a webbed wiring pattern for supplying the video or driving signal from the output electrode with respect to an electrode section (not shown) for applying a voltage to the liquid crystal layer 126 which is provided on the inner surface of the glass substrate 121. There is electrical conduction by an anisotropic conductive film 118 between the plated layer 113 and the connecting section 124, and between the IC chip 122 and the connecting sections 123 and 124.
Further, as shown in FIG. 6, the flexible wiring board 110 is bent 90° at a position slightly shifted outward from the end of the liquid crystal panel 120 in a direction (downward in FIG. 6) which would make the distant end of the flexible wiring board 110 from the liquid crystal panel 120 to approach the liquid crystal panel 120 with the side of the flexible wiring board 110 provided with the plated layer 113 facing inward.
Note that, FIG. 6 shows the arrangement where the base polymer film 111 and the copper foil pattern 112 are bonded by the adhesive (copper foil adhesive layer 115), yet the arrangement where the base polymer film 111 and the copper foil pattern 112 are directly bonded with each other without using the adhesive, i.e., an adhesive-less flexible wiring board has been used as well.
The conventional flexible wiring boards as shown in FIGS. 5 and 6 normally employed the insulative protecting films 104 and 114 having the same thickness as that of the base polymer films 101 and 111 without particularly using the insulative protecting films 104 and 114 having a thinner thickness than that of the base polymer films 104 and 114.
The inventors of the present invention have once used a polymer film having a thickness of 25 μm as the base polymer films 101 and 111 or the insulative protecting films 104 and 114. This was due to various drawbacks with the polymer film having a thickness thinner than 25 μm, such as difficulty in handling such a film due to its thickness, which causes wrinkles to generate and leads to misalignment, exposing the wiring which should not be exposed.
In order to improve the flexibility of the flexible wiring board, study was made by the inventors of the present invention to make the thickness of the polymer film (base polymer films 101 and 111 or insulative protecting films 104 and 114) thinner than 25 μm.
To this end, the inventors of the present invention first prepared a flexible wiring board only as the base polymer films 101 and 111 having a thickness of 12.5 μm, thinner than 25 μm. This was because the base polymer films 101 and 111 as prepared by the inventors of the present invention were bonded with the copper foil and therefore they were relatively easy to handle even with the thickness thinner than 25 μm, whereas the insulative protecting films 104 and 114 need to be handled as the film and become difficult handle when the thickness is made thinner than 25 μm.
Despite this, the flexible wiring board thus prepared was very susceptible to wire breakage of the copper foil pattern 112 when the flexible wiring board was bent to mount it on the liquid crystal panel 120. Accordingly, the yield of the liquid crystal display device was significantly low. The same problem was found to exist also for the insulative protecting films 104 and 114 having the same thickness as that of the base polymer films 101 and 111 prepared.
This problem is caused by the following mechanism. When the insulative protecting films 104 and 114 having the same or thicker thickness than that of the base polymer films 101 and 111 are used, by the rigidity of the insulative protecting films 104 and 114, there generates a large difference in rigidity between a portion of the flexible wiring board covered with the insulative protecting films 104 and 114, and a portion not covered with the insulative protecting films 104 and 114. As a result, bending stress when the flexible wiring board is bent concentrates on a boundary portion (boundary portion 117 in FIG. 6) between the insulative protecting films 104 and 114, and the plated layers 103 and 113 with the likely result of wire breakage of the copper foil pattern 112 running in a direction perpendicular to the line of bend (bend line).
Thus, the conventional flexible wiring board has the problem of low wiring reliability.
In order to solve this problem, for example, Japanese Unexamined Utility Model No. 70630/1992 (Jitsukaihei 4-70630) (published date: Jun. 23, 1992) proposes a structure wherein a copper foil pattern is exposed at the portion where the flexible wiring board is bent by removing the insulative protecting film therefrom, and the copper foil pattern thus exposed is plated to prevent rust.
However, in this structure, even though the copper foil pattern at the bending portion is plated to prevent rust, the copper foil pattern is not covered with the insulative protecting film over a wide range from a region in the vicinity of the connected portion to the region where bending is made, and thus the copper foil pattern is not protected both electrically and chemically over this area. Thus, the copper foil pattern may not be shielded efficiently over the unprotected area, or wire breakage of the copper foil pattern may generate.
Further, since the flexible wiring board is bent at the plated portion, in the case where plating is made by Au/Ni plating, which offers superior connection stability, it becomes difficult to bend the copper foil pattern by the rigid Ni film and the copper foil pattern becomes susceptible to cracking. Thus, wire breakage is more likely to occur when bent and reliability may suffer contrary to the intended purpose.
Further, for example, Japanese Unexamined Patent Publication No. 138387/1997 (Tokukaihei 9-138387) (published date: May 27, 1997) proposes preventing wire breakage in a circuit pattern by making the end portion of the insulative protecting film to have a wave so as to disperse the stress applied on the boundary of the insulative protecting film and the plated portion when bent.
However, because this structure employs forming a wave on the insulative protecting film, manufacture of the flexible wiring board becomes complex, resulting in inefficient manufacture and increased manufacturing cost, and also the dimension of the flexible wiring board is increased for the height of the wave formed on the insulative protecting film.
Further, for example, Japanese Unexamined Patent Publication No. 92480/1995 (Tokukaihei 7-92480) (published date: Apr. 7, 1995) proposes an arrangement of a flexible wiring board having circuit wiring on the both sides of the flexible substrates, in which an insulative protecting film (film cover lay) is pasted on one surface of the flexible substrate using an adhesive, and on the other surface of the flexible wiring is applied, instead of the insulative protecting film, liquid resin (thermosetting resin) such as liquid polyimide resin (polyimide ink) or resist (resist ink) which is cured thereon so as to form an insulative protecting film (ink cover lay).
According to this arrangement, using ink cover lay which is more flexible than film cover lay, the surface on which the cured liquid resin is formed is connected to a display panel and the connected surface is bent in a direction to face inward. As a result, compared with the arrangement where the insulative protecting films are pasted on the both surfaces, less stress concentrates at the boundary of the cover lay and the plated portion and wire breakage of the circuit wiring can be prevented to some degree.
However, since the ink cover lay is formed by applying liquid resin, it is susceptible to nonuniformity. Especially, because the conductor pattern such as a copper foil which is formed as the circuit wiring includes a large number of “ribs” which are protrusions on the surface of the flexible substrate and because there are level differences between the surface of the flexible substrate and the protrusions of the ribs, the liquid resin may not be applied on the side surface of the ribs. Thus, the foregoing arrangement has the problem of insulation failure especially at the side surface of the ribs of the conductor pattern.
Further, the liquid resin used to form the ink cover lay includes epoxy resin or polyimide resin (liquid polyimide), and the ink cover lay which is made of cured resist has poor insulation and poor reliability (certainty of offering insulative and physical protection is poor). Also, since the curing temperature of polyimide is high, heating at high temperature is required. Thus, with the flexible wiring board in which the copper foil pattern (wiring) is pasted by the copper foil adhesive, the copper foil adhesive, which generally has inferior heat resistance, deteriorates. This sets a limit to the use of polyimide resin to an expensive adhesive-less flexible wiring board. That is, the ink cover lay using polyimide resin has limited use compared with the insulative protecting film which is pasted by the adhesive.
Note that, in recent years, there has been a demand for further reducing the weight of a module, for example, in liquid crystal panels. To this end, there has been active research on use of thinner glass substrates or thinner plastic substrates to reduce the weight of the substrate which takes up a large proportion of the total weight. In view of these backgrounds, there is need for a flexible wiring board with smaller bend radius while maintaining high reliability.